1. Field of the Invention
The present invention relates to the field of measuring systems, and more particularly to a load cell device used in a measuring and detection system for determining the weight and therefore the quantity of a liquid in a tank subjected to external forces.
2. Description of the Related Art
The present invention discloses a load cell device which can be used in a measuring system similar to that described in U.S. Pat. No. 5,133,212, and assigned to the assignee of the present invention. That system teaches the use of a strain gauge system which can determine the weight of the contents of a tank and therefore the quantity of liquid in the tank without the need for sensors or other measuring devices inside the tank. One common application of the present invention is for use in measuring the amount of liquid and other matter inside an airplane waste tank.
Waste tanks designed into most aircraft models have liquid level monitoring and control devices which operate by sensing the waste contents in some way. Most sensors require access to the inside of the tank. These sensors are xe2x80x9cintrusivexe2x80x9d into the tank, and by their nature must come into contact with the tank""s contents. These type of intrusive devices are undesirable in many cases because they may easily corrode or become fouled in the presence of the waste material. This corrosion or fouling often renders the intrusive sensors unreliable and inoperable.
U.S. Pat. No. 5,133,212 describes a system which is able to measure the level of liquid inside the waste tank without the use of intrusive sensors. In the ""212 patent, the waste tank is weighed using a load cell device. The load cell utilizes strain gauges to measure the amount of force (and hence the weight) exerted by the tank and its contents. The output of the load cell device is an electrical signal which is proportional to the amount of force exerted by the tank. The weight of the empty tank (which is a known quantity) is then subtracted from the measured weight. The weight of the waste amount is indicated by means of a scaled electrical signal. The liquid level is then easily calculated.
The signal from the load cell is also filtered to avoid changes due to air turbulence or oscillating motions of the airplane. In practice, a plurality of load cell devices are used for each waste tank, in order to provide the most accurate measurement of the liquid level.
Most larger passenger airlines in operation today use a cylindrically shaped tank. However, it is clear that the present invention is not intended to be limited to any particular shape of waste tank. In one aircraft configuration, the tank is disposed vertically within the aircraft structure. A flange on the tank is used to support the tank and its contents. Typically, the flange is manufactured from a non-metallic reinforced fiberglass type material. The flange usually rests on a support structure made of aluminum metal.
The flanges are attached to the support structure by means of bolts. In actual practice, a pair of bolts is used with each flange. It is known in the prior art, that a load cell can be placed in between the flange and the support structure. The load cell is held in place with the bolts used to secure the tank. The load cells transduce the weight-generated forces as is described above.
There are limitations with the method and apparatus as described above. In particular, it is known that the fiber-glass flanges have a very uneven surface. This is due to the methods which are used to manufacture the flange. (At other times, the surface of the support structure may also be uneven.) Because most airplanes already have the waste tanks in place, it is not practical to attempt to even out the surface of the flange.
It has also been found that when a load cell is bolted between the flange and the support structure, uneven and large forces are exerted on the load cell. These forces are primarily in the form of a turning moment. The magnitude and direction of the moment cannot be readily determined beforehand because of the uneven nature of the flange. This turning moment interferes with the force sensing of the load cell.
In addition to the uneven surface, the flange material is typically much less stiff than the aluminum support structure. When the tank is filled, the changes in the rotational moment at the sensor adds to the signal output. The extra signal component is due to the fact that any sensor cannot totally cancel the unwanted forces caused by the moments. The flange stiffness can vary from one flange location to another. The stiffness can also vary at each location over the life cycle of the component. These factors make it difficult to calibrate the system and retain the required accuracy through the life of the unit.
It is therefore an object of the present invention to provide a load cell device which can be used in connection with waste tanks on existing aircraft.
It is another objection of the present invention to provide a load cell device which accurately measure the weight of the waste tank despite the movement or attitude of the aircraft structure.
It is yet another object of the present invention to provide a load cell device which can compensate for turning moments exerted on the device due to any irregularities or uneven surfaces where the load cell device is mounted.
The present invention consists of a load cell device which can be used in a system to measure the weight of airplane liquid waste tanks. The load cell assembly is designed to minimize unwanted forces and turning moments acting on the strain gauge devices which actually measure the weight of the tanks. Elimination of these unwanted forces and moments results in a more accurate measurement of the weight of the tank.
The load cell assembly of the present invention consists of an upper beam and a lower beam which are coupled together. A spherical bearing is placed between these two elements. The assembly is held together by a fastener bolt which passes through the spherical bearing, permitting the elements limited rotational movement about an axis coaxial with the fastener bolt. The lower beam has attached to it one or more strain gauge devices which are used to measure the vertical load acting on the load cell device.